Add unit test of smt2_convt::convert_identifier

[thomasspriggs]

So that the proper working of this function can be checked
independently from the rest of the smt2 support.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6225 (665eaad) into develop (141f3ce) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6225      +/-   ##
===========================================
+ Coverage    75.30%   75.46%   +0.15%     
===========================================
  Files         1458     1459       +1     
  Lines       161436   161444       +8     
===========================================
+ Hits        121575   121838     +263     
+ Misses       39861    39606     -255     

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_conv.h	100.00% <ø> (ø)
unit/solvers/smt2/smt2_conv.cpp	100.00% <100.00%> (ø)
src/ansi-c/ansi_c_declaration.cpp	72.89% <0.00%> (+0.93%)	⬆️
src/langapi/mode.cpp	92.85% <0.00%> (+2.38%)	⬆️
src/ansi-c/parser_static.inc	96.58% <0.00%> (+2.56%)	⬆️
src/linking/static_lifetime_init.cpp	98.63% <0.00%> (+2.73%)	⬆️
src/ansi-c/parser.y	79.24% <0.00%> (+16.85%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 665abf7...665eaad. Read the comment docs.

[martin-cs]

Great stuff! Thank you.